- OSX likes to turn random Unicode symbols into emoji (extends past Canvas)
- Fractional font sizes on Chrome/Linux get bizarrely stretched out (after a transform that makes the font not a fractional size)
- Complex clipping operations freak Chrome out
- All the compositing rendering options are broken somewhere or mis-defined to the point that it's weirdly hard to render anything to an alpha channel
You also need input APIs to make a game, and the input APIs often have strange inconsistencies in function names, returned objects, or event ordering.
Canvas is nice and straightforward if you want a purely sprite-based game with only mouse/keyboard input (but even then you need to jump through hoops if you want pixel art). Anything beyond that is going to need cross-platform debugging.
From what I know, canvas rendering is pretty much the same across different browsers. It's a pretty low level API so there's not as much room for interpretation. In any case, the Acid3 test you're running is focused on the specifics of HTML/CSS layout, and don't think it tests the canvas at all.
I think that canvas has done a good job of taking over the use cases that Flash had, and is very portable these days (basic canvas has 98-99% support[0]).
[0] https://caniuse.com/#feat=canvas